The Austrian man who allegedly imprisoned his daughter and fathered seven children with her first planned to build his secret cell as early as 1978, when his daughter was 12, authorities said Monday.
Investigators said a total of eight doors fitted with sophisticated locks and electronics secured the underground warren of windowless rooms where they say Josef Fritzl held his daughter captive for two dozen years.
"This was not built from one day to the next," said police Col. Franz Polzer, who is overseeing the investigation into a case that has stunned Austria and the world.

An Austrian police handout picture released April 28, 2008 shows a man suspected of keeping his daughter prisoner and abusing her for 24 years in the basement of a house in the small Austrian village of Amstetten.(Xinhua/Reuters Photo) He said the main door weighed about 500 kilograms (half a ton).
Polzer said an investigation showed that the apartment complex owned by Fritzl originally was built in 1890, and that he applied for permits to expand it in 1978. He said police believe the plans for that expansion included the secret rooms because adding them later would have been far more difficult and expensive.
Prosecutors told reporters in Amstetten, Fritzl's hometown about 75 miles west of Vienna, that they will have their first meeting with the 73-year-old suspect on Wednesday or Thursday.
Officials said Fritzl's wife, the daughter he allegedly imprisoned and raped, and the children born of the illicit relationship are learning how to live together as they get psychiatric care and counseling.
Source: Xinhua/Agencies